It's a difficult one. Yes his hand appears to go in a bit deeper and higher than was strictly necessary but I don't think it's actually that unusual on this show. What has drawn our attention to it is her reaction. We probably wouldn't have looked twice otherwise. The directors obviously tell them to cuddle up to their partner, look affectionate and mutually supportive etc. when the camera is on them, and he clearly thought their 'friendship' was far closer than she thinks. He obviously has to have his hands on her all the time in practise sessions and on stage, and I think he just forgot his boundaries for a moment, or misjudged them. I imagine there's lots of endorphins and adrenalin flying around.
I saw her rejecting his high five as well. I don't watch every week so I've no idea if this really is a 'running joke' or not, but her face said otherwise to me. He clearly gives her the ick. She doesn't like him. As far as physical touch goes, she's doing what's required of her professionally, but absolutely nothing more. Any kind of PDA for the camera once off the dance floor, nah. She's not having it.
Realistically you aren't going to get on famously with everyone you are partnered with, there won't always be an easy chemistry, but you have to keep up the pretence for the sake of the show. Katya is barely managing that here.
I think Strictly has had its day. There is too much of this sort of drama and constant over-analysis of everyone's body language and behaviour now. If it was ever once a feel-good show full of wholesomeness, it's now constantly mired in scandal. Accusations of infidelity, bullying or misogyny, backstabbing, he said/she said nonsense. Once that starts to dominate, you can't see any of these supposed 'great friendships' forged between dancing partners as anything other than either sexual, seedy, exploitative, or just completely fake and contrived.
The men involved (both professional and contestants) are going to start questioning if it's worth it, given the risk they run of of being accused of something or other. Made out to be a complete sleaze, or just humiliated on TV by someone who obviously despises you.